Kevin, I think it would be a good idea to start plugging up every vacuum port one by one until vacuum is normal. That should give you a good idea of which one it is. Start with the big lines since it seems like it would be a pretty big leak.
Also, get another vacuum tester and make sure that yours is accurate or use the one you have on a friend's Mustang to see if it registers what you would expect.
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351W 89 Mustang GT Convertible